1. Field of the Invention.
The present invention relates generally to improvements in oil and gas well perforating devices, and more particularly, but not by way of limitation, to an improved perforating device which is able to function reliably under high hydrostatic pressures of very deep wells while remaining easily retrievable.
2. Discussion of the Prior Art.
Following the drilling of a typical oil or gas well, the well is completed by installing a well liner or casing, which is cemented into place, primarily to prevent the possibility of petroleum produces from entering and contaminating wafer bearing strata which may lie between the oil bearing strata and the surface. After this casing is in place, it is usually necessary to perforate the casing to enable fluids from the oil or gas bearing formations to flow into the well bore. The perforations are usually made by detonating a specially configured explosive charges which produce jets of extremely high pressure gases. Each high pressure jet blows a hole through the well casing and creates a fracture extending for some distance into the surrounding formation.
The explosive charge used for this purpose is commonly shaped into the form of a hollow cone to take advantage of the Munroe effect, which achieves remarkable penetration by focusing the explosive effect on one spot, much as a magnifying glass focuses the sun's rays. A single perforator device usually includes a plurality of explosive charges for making several casing perforations simultaneously.
Since the penetration ability of the perforator is highly dependent upon this hollow conical shape, it is necessary to protect the explosive charges, as well as the detonating cord, the detonators and the electrical wiring from the well bottom environment. The harshness of these environmental conditions is considerably exacerbated by the increasing depths to which current wells are drilled.
To provide adequate protection a pressure vessel (a perforating gun body) is usually provided to enclose the explosives, detonators, detonating cords and electrical wiring. The body of a retrievable perforating device, which serves as a pressure vessel, must have the following characteristics: (1) sufficient structural strength to resist the hydrostatic pressure in the well bore without collapsing since, as well depths increase, there is a corresponding increase in hydrostatic pressure; (2) sufficient structural strength to contain the effects of the explosions that occur when the perforating device is fired in order to protect the well casing from unnecessary damage; (3) confinement capability to contain the debris from the explosive charges to avoid contaminating the well; and (4) sufficient dimensional stability before and after detonation to allow the perforating device to be retrieved from the well bore.
As wells are drilled to ever deeper depths in the search for producible hydrocarbons, the tubular goods (casing, liner and tubing) have changed. To withstand the stresses caused by increased well pressures and higher tensile stresses due to longer strings of tubular goods, the trend is toward casing, pipe and tubing with thicker walls. Since the outside diameters of the tubular goods are held constant to conform to standard sizes, the increase in wall thickness means a reduction in inside diameter of the tubular goods.
Also, in the interest of maximizing well production rates, the trend has been toward larger perforating explosive charges to achieve higher performance perforations, i.e. larger hole sizes in conjunction with deeper penetrations. As expected, this requires dimensionally larger explosive charges with more explosive power, and this has lead to greater deformation of the perforating device about its circumference at the points of penetration. Retrieval of prior art perforating devices from deep wells following perforation by such larger explosive charges in close fitting tubular goods has often not been possible by the usual cable pulling technique because of the wall jamming between the perforating device and the surrounding tubular goods.
There has been a need for a perforating device that can deliver a larger more powerful explosive charge through tubular goods that have smaller internal diameters while avoiding the jamming interference between the perforated wall of the perforating device and the surrounding tubular goods.